1. Field of the Invention
This invention relates to devices for placing a looped belt under tension and more particularly to such a device that is inexpensive and can be employed in multiple instances to apply proper tension to all of the belts of a complex machine such as a photocopier or medical apparatus. The invention relates also to a method of driving a looped belt in a mode of tensioning that is selectively static or dynamic.
2. Description of the Prior Art
Many complex machines such as photocopiers and sophisticated medical apparatus employ one or more belt drives for turning pulleys and the axles on which they are mounted. The circular movements of the pulleys and axles are converted as may be required into rectilinear and other types of movement, which may be continuous or intermittent.
The belts must all be placed under proper tension in order to work properly. If a belt is too tight it tends to cock pulleys around which it is trained relative to the axles that respectively support the pulleys and to bend the axles. If a belt is too loose it slips on the pulleys and does not turn them reliably, so that the apparatus malfunctions.
Belts tend to stretch during use, so that a belt that is under proper tension when installed may later be too loose; and during operation of a machine, vibrations are often set up that change the tension on a belt many times a second.
If a belt is trained around pulleys that rotate about respective axes that are all fixed (static mode of operation), stretching can be compensated for by stopping the machine periodically and repositioning the pulleys to take up the slack. But this entails machine downtime and is expensive in terms of labor and lost output. Vibrations must simply be accepted in the static mode of operation.
If a belt is trained around pulleys at least one of which rotates about an axis that can be translated in a direction having a component normal to the axis (dynamic mode of operation), belt stretch can be compensated for automatically by a spring that gradually displaces the pulley's axis of rotation to take up the slack that would otherwise develop. The same spring can absorb the vibrations, thereby keeping the tension on the belt substantially constant. This type of mount is likely to be costlier than a static mount.
While pulley mounts are not individually expensive, their total cost in a complex machine is not inconsiderable. Typically each mount is individually designed in view of available anchor points, etc., in a machine. This entails the cost of design and engineering to determine the best mount design at each location in a machine, the cost of manufacturing to the several design specifications, and the cost of familiarizing service personnel with the several designs.
All of the problems described above are compounded by the many different designs of machines that have belt drives. Each model of each kind of machine requires a separate solution to the problem of mounting each belt. These costs contribute considerably to the total cost of the machines.